1. Field of the Invention
This invention relates to conveying sheet material through a printing device and, more particularly, a printing device having a pivotably restricted paper guide for accurately guiding cut sheet paper having various thicknesses toward a nip of a nip roller pair.
2. Description of the Related Art
Printing devices such as ink jet printers or thermal printers have a main shaft that includes one or more coaxial drive rollers and a plurality of pinch rollers to define a paper transport path between the pinch rollers and the one or more drive rollers. Cut sheet or continuous paper is generally transported over about half the circumference of the one or more drive rollers so that a printing side of the sheet material is exposed to a printing device such as an ink jet or a thermal printer. One example of such a device can be found in U.S. Pat. No. 4,729,557 to Kiyohara.
In order to maintain an adequate degree of frictional contact between the advancing sheet material and the one or more drive rollers, prior art devices employ resilient guide members or spring members to directly or indirectly press the sheet material into contact with the drive roller with the sheet material sandwiched between the pinch and drive rollers during transport. The spring members must apply a strong enough force to the sheet material such that adequate friction is maintained between the drive roller and the sheet material. The force applied to the sheet material by the spring or the guide member, however, must not be so strong that thin sheet material cannot overcome the force applied by the spring member. Moreover, spring members that apply a force that is too strong will cause a thin sheet material to jam within the printing device.
One solution as set forth in U.S. Pat. No. 4,729,557 is to provide relatively weak spring members that are upon engagement even with relatively thin sheet material. However, in the event of printing a relatively thick (or stiff) sheet material, the relatively weak spring, which is sufficient for guiding a relatively thin sheet material, fails to provide adequate force to the thick material such that proper amount of friction is maintained between the thick material and the drive roller. In addition, relatively thick sheet material includes an inherent rigidity such that as the relatively thick material is guided around the main drive rollers, the relatively thick material has a tendency to bend the relatively weak spring to an undesirable extent. This results in paper jams for thick sheet material because the leading edge of the thick material is not properly guided to an outlet or an acceptable position relative to a nip roller pair located adjacent the outlet of the printing device.